The effect of modulated feed on liquid hold-up and pressure drop in trickle bed reactor

The performance of a trickle bed reactor can be considerably affected by the liquid amount and uniformity of its flow through the catalyst bed. The aim of the work was to determine the influence of periodic modulation of liquid feed on hydrodynamic properties of the pilot-scale trickle bed reactor using the method of residence time distribution experiment. By evaluation of measurement, mean residence time, effective dynamic liquid hold-up and Peclet number of longitudinal mixing were determined. Simultaneously, freely drained-out dynamic hold-up of liquid and pressure drop in packed bed were estimated and the results were compared with continuous flow measurement. The experiments with the system water - air were made with both continuous and periodically modulated feed of liquid. A glass trickle bed reactor of length 500 mm and 106 mm in diameter was filled with 4.6-mm active carbon extrudates or glass beads 5 mm in diameter. Evaluation of liquid conductivity response in the catalyst layer to injection of a saturated KCl solution was performed by the moment method and by identification of parameters of the continuous stirred tank cascade model. It was found that periodic liquid feed rate modulation leads to a decrease in the freely drained-out liquid and also in effective dynamic hold-up of liquid in the bed. In case of liquid feed rate modulation a minimum was observed on the dependence of liquid hold-up at a split of 0.5 and the time period corresponding to the theoretical mean hold-up time, calculated from the flow rate of liquid and void volume in the packed bed. In the periodic feed rate modulation regime under the same hydrodynamic conditions the values of the Peclet criterion increase compared with continuous flow. Suppression of axial dispersion in the periodic regime leads to approaching to the ideal plug flow.